1. Field of the Invention
Embodiments of the invention are generally directed to the field of optical systems; particularly to nonsymmetric imaging optical systems and, more particularly to nonsymmetric imaging optical systems having two or more phi (φ)-polynomial surfaces, and methods for designing such nonsymmetric optical systems.
2. Related Art Discussion
Starting in the 1960s, motivated by the advance in LWIR detectors and the accompanying need for stray light control, a number of reflective, unobscured optical systems were designed, particularly, as concept designs for missile defense. While many of these systems appear to lack rotational symmetry, detailed analysis reveals that any successful design with a significant field of view was in fact based on a rotationally symmetric design with an offset aperture, a biased field, or both. This fact could be anticipated, as many systems that depart from rotational symmetry immediately display on-axis coma, where the axis for a nonsymmetric system is defined by the optical axis ray (OAR). While there are special configurations that eliminate axial coma, there are very few practical forms that do not reduce to a rotational symmetric form.
An example of a reflective, unobscured optical system is disclosed in Rodgers U.S. Pat. No. 5,309,276 which had the property of providing the largest planar, circular input aperture in the smallest overall spherical volume. The particular form embodied in the '276 patent is shown in FIG. 3. This optical design is a 9:1 afocal relay that operates over a 3° full FOV using four minors, and provides a real, accessible exit pupil, which was often a requirement in earlier infrared systems requiring cooled detectors. In use, it is coupled with a fast f/number refractive component in a dewar near the detector. It is based on using off-axis sections of rotationally symmetric conic minors that are folded into the spherical volume by using one fold mirror (mirror 3).
Many applications would exploit a larger FOV if it were available with usable performance. In addition, if an optical form could be developed at a fast enough f/number while maintaining a small envelope diameter, it becomes feasible to transition to an uncooled detector, thereby abandoning the need for the reimaging configuration, the external exit pupil, and the refractive component in the dewar. One method to achieving faster f/numbers is to implement a nonsymmetric optical surface.
When the symmetry constraint is removed, the traditional aberrations (spherical, coma, and astigmatism) develop a multi-nodal field dependence where there may now be multiple points in the FOV where a specific aberration type may go to zero. The seminal example is binodal astigmatism, first recognized by R. V. Shack, K. P. Thompson, “Influence of alignment errors of a telescope system,” Proc. SPIE 251, 146-153 (1980). However, due to the fact that any tilted and decentered optical system with rotationally symmetric parent surfaces could not be corrected for axial coma, this theory has previously only been useful during the optical design of offset aperture and/or field biased optical systems.
Thompson, in 2005, described the new aberration field dependencies that arise in nodal aberration theory using a new display, the full field aberration display (K. P. Thompson, “Description of the third-order optical aberrations of near-circular pupil optical systems without symmetry,” J. Opt. Soc. Am. A 22, 1389-1401 (2005)). The theory developed by Thompson is limited to tilted and decentered optical imaging systems made up of rotationally symmetric components, or offset aperture portions thereof. Recently, Schmid et al. (T. Schmid, J. P. Rolland, A. Rakich, and K. P. Thompson, “Separation of the effects of astigmatic figure error from misalignments using Nodal Aberration Theory (NAT),” Opt. Express 18, 17433-17447 (2010)) combined a nonsymmetric surface placed at an aperture stop with nodal aberration theory. With this new result and new fabrication methods where non-rotationally symmetric optical quality surfaces (known as φ-polynomial surfaces) can be diamond turned, the optical designer is now able to target the third-order aberrations (spherical, coma, astigmatism) and their nodal behavior during optical design using tilted φ-polynomial surfaces to create high performance imaging systems with no particular symmetry constraints.
The inventors recognize that solutions to the problems and challenges of designing and fabricating unobscured, truly nonsymmetric optical systems would be advantageous and beneficial. Moreover, the identification and use of a new fabrication degree of freedom enabled by the introduction of slow-servos to diamond machining, to provide faster optical systems with wider fields of view and minimal envelope size would provide further benefits and advantages.